When cutting flat metal plates with a flame cutting torch, or other cutting head, the head is moved along the upper surface of a plate and is maintained at a preselected distance from the plate to obtain optimum cutting parameters, such as fuel consumption, cutting speed and quality of the cut. Optimization of these parameters require that the end of the torch be maintained at a preselected gap or spacing from the top of the plate being cut; however, the plate is not uniform, the cutting is performed in a dirty atmosphere and surface impediments or obstacles may exist in the cutting path which could damage the torch if it is driven into the obstacle. In addition, under normal circumstances, several cutting heads are operated in unison from a single support beam above the workpiece. Each of the heads must be adjusted independently as different portions of the workpiece are being cut by the individual cutting heads or flame torches. Consequently, control of a group of cutting heads or flame torches cutting several identical cuts in a workpiece places tremendous burden upon the operator. The operator must anticipate the desired optimum spacing of each individual cutting head which may include six or more individual heads, and adjust each head separately in a vertical direction as the heads are moved along the workpiece. This presents a monumental control problem which defies manual manipulation skills even though each of the cutting heads have their own individual motors for adjusting the vertical height of the head with respect to the workpiece. Each of these motors is controlled by a separate unit which the operator must manipulate for the purposes of maintaining the optimum flame length or operating gap for the cutting heads as they progress along the upper surface of the plate being cut.
An optimum gap for a cutting torch is relatively close to the upper surface of the workpiece and is determined by the material of the plate, the equipment being employed, the fuel settings and the speed at which the cutting heads are manipulated along the upper surface being cut. Such an optimum small operating gap cannot be easily maintained. Often poor quality cuts are created. In many instances, the head is stubbed or spaced away from the workpiece sufficiently to lose the flame and, thus, interrupt the cut, with the obvious disadvantages of this eventuality, especially if several cuts are being made at the same time. Manual manipulation is complicated by the inability to observe the actual conditions being experienced by the individual cutting heads and to manipulate several heads at the same time. All of these disadvantages have resulted in a somewhat normal practice of setting the operating gap at a large distance, increasing the fuel consumption, slowing down the cutting rate, and hoping that no obstacles appear in the path of the moving torch. In this fashion, an inefficient cutting is accomplished with a large operating gap which has only the advantage of facilitating large variations in the plate surface without actual destruction of the torch, interrupting the cut, or extinguishing of the flame.
To overcome these disadvantages, for many years efforts have been made to sense the spacing between the plate and the head for the purpose of automatically controlling the operating gap by using the reversible motor through a feedback arrangement as opposed to manipulation of the vertical heights of the cutting heads by manual operation of the adjustment motors. One attempt involves an air back pressure arrangement as shown in Wirth U.S. Pat. No. 3,746,326. At best, this produced an adjustment having a tolerance of 0.25 inches, which is not generally close control of the operating gap. In addition, the heat, metal spatter, hot material, dust and bad environment seriously hindered the ability of the air sensor to provide accurate feedback control over the vertical height of the cutting torches. Another attempt involved the use of a capacitor ring surrounding the cutting torch and parallel to the workpiece. As the capacitance changed, a feedback signal was created that adjusted the height of the torch. However, this type of arrangement involved very small changes in the sensed capacitance. Such small changes were difficult to recognize and changed substantially due to the heat, dust and other capacitive affecting material between the sensing plate and the upper surface of the workpiece. These particular attempts to automatically control the operating gap or flame length of a cutting torch as it moved along a flat plate were not successful and were relatively expensive.
To produce a less expensive arrangement, it was suggested that limit switches could be employed between the cutting head, or torch, and the workpiece. One switch could determine the minimum height and another torch could determine the maximum height, or a single limit switch could be employed. These attempts were substantial disasters. Mechanical limit switches were quickly broken or destroyed by slag and other abutments on the plate being cut. Further, these limit switches, as the air back pressure arrangement, could anticipate changes in vertical height only in a single direction. Since the torch could move in several directions, limit switches were substantially ineffective even if they had been capable of withstanding the rigors of the adverse environment, which they were not.
In summary, attempts to automatically control the operating gap of the cutting torch to an optimum value by mechanical, pneumatic, and electrical feedback arrangements have been unsuccessful in that they are expensive, incapable of close control and subject to damage by the adverse environment. Consequently, manual operation is now quite prevalent even though several attempts have been made to automatically control the operating gap.